Hello
I have a domain with 1 & 1 and they are telling me that I MUST have reverse DNS setup but the thing is, I already have accounts setup on the same server that are working fine.
I emailed one and one and told them this, although they told me that the domain wouldn't work without reverse DNS so they would not let me change the name servers
Can someone tell me how I can set this up please so I can get my accounts working

Thanks

You need to speak with your host on this issue. The owner of the IP block would control the reverse DNS entries for your IP. They may need you to submit a request to change the entry, or may have some of of interface or arrangement (ie. CNAME PTRs) where you can change it yourself.